This invention relates to a field control system of induction machines which is stable in operation and economical in manufacture.
Heretofore an arrangement as shown in FIG. 1 has been frequently used for the field control of induction machines, the arrangement comprising a main power circuit and a control circuit. The main power circuit includes a DC power source 1, electric valves 2A and 2B for chopping electric current supplied from the DC power source 1, bypass rectifiers 3A and 3B cross-connected between the electric valves 2A and 2B, a reactor 4 one end of which is connected to the electric valve 2A, a thyristor 5 connected in parallel with the reactor 4, a thyristor bridge 6 connected through the reactor 4 and the thyristor 5 to the electric valves 2A and 2B, and a current detector 7 connected between the electric valve 2B and the thyristor bridge 6. An induction machine 8 is connected to the AC side of the thyristor bridge 6. The running speed of the induction machine 8 is controlled with respect to its slip frequency as described hereinbelow.
In the control circuit, a pulse generator 9 is directly coupled to the induction machine for generating pulses in proportion to the running speed of the induction machine 8. The frequency of the output of the pulse generator 9 is converted by a frequency-voltage converter 10 into a voltage V.sub.N proportional to the running speed of the induction machine 8. An amplifier 11 is connected to amplify the difference between the voltage V.sub.N and a reference signal S.sub.R indicative of the running speed of the induction machine 8, and the output of the amplifier 11 is applied to a function generator 12 as a torque standard. The function generator 12 thus generates a standard signal I.sub.R related to the primary current of the induction machine 8 inclusive of the exciting current, and this signal I.sub.R is compared with a signal I indicative of an inverter current obtained from the current detector 7. A phase shifter 13 is connected to shift the phase angle of the difference signal. A pulse amplifier 14 is connected to amplify the output signal PWM from the phase shifter 13. The output signal SWG obtained from the pulse amplifier 14 is applied to the control gate electrodes of the electric valves 2A and 2B.
The output I.sub.2 constituting a torque standard is also amplified by an amplifier 15. A voltage-frequency converter 16 is connected to convert the output of the amplifier 15 into a slip-frequency indicative signal f.sub.S which is added (in the case of motor-operation of the induction machine) to or subtracted (in the case of regenerating operation of the same machine) from the pulse frequency f.sub.N of the pulse generator 9, the sum or difference being amplified in a pulse amplifier 17. The output of the pulse amplifier 17 constituting SCR.sub.G timing signal for the inverter operation is applied to the control gate electrodes of the thyristor bridge 6 and the thyristor 5.
For the purpose of effecting a reduced flux control (which may otherwise be termed a weakened field control) of the induction machine, a transformer 18 is provided to detect terminal voltage of the induction machine, the output of the transformer 18 being converted into a DC voltage V.sub.M by a rectifier 19. The voltage V.sub.M is compared with a reference voltage V.sub.R, and the difference is amplified by an adder-amplifier 20. The output of the amplifier 20 is further added to the aforementioned signal I.sub.2, of the torque standard (or secondary current standard) and the sum is amplified by the amplifier 15. With the aforementioned circuitry, when the terminal voltage V.sub.M of the induction machine 8 exceeds the voltage standard V.sub.R, the output of the amplifier 20 increases, thereby increasing the slip frequency f.sub.S for effecting the reduced flux control.
In the conventional field control system utilizing the above described control circuit, since a large amount of high frequency components are contained in the terminal voltage of the induction machine, the DC voltage V.sub.M obtained by rectifying the terminal voltage widely differs from the induced electromotive force of the induction machine, thus giving rise to a disadvantage which causes the reduced flux control of the induction machine to be substantially impossible.